Let's simply start our clock at t = 0 s. I am using a coordinate system where the origin is the point where the car and cop are at t = 0 s, and choose the +x direction in the direction of the velocity of the car.

So for the car:

<-- This is constant.

The position of the car is given as

For the cop:

The position of the cop is given as

We want to find where the car and cop meet, so equate these two expressions:

Now solve for t.

I get (which we already knew) and

It is possible that the computer didn't accept your second (and correct) answer because you didn't give the result to the proper number of significant digits: t = 22.7 s.

-Dan